Improve case insensitive search to avoid allocations. (#4394)

* Improve case sensitive search to avoid allocations.

```
❯ benchcmp  before.txt after.txt
benchmark                                            old ns/op     new ns/op     delta
Benchmark_LineFilter/default_true_(?i)foo-16         2400          2233          -6.96%
Benchmark_LineFilter/simplified_true_(?i)foo-16      201           228           +13.13%
Benchmark_LineFilter/default_false_(?i)foo-16        2443          2376          -2.74%
Benchmark_LineFilter/simplified_false_(?i)foo-16     185           231           +24.96%

benchmark                                            old allocs     new allocs     delta
Benchmark_LineFilter/default_true_(?i)foo-16         0              0              +0.00%
Benchmark_LineFilter/simplified_true_(?i)foo-16      1              0              -100.00%
Benchmark_LineFilter/default_false_(?i)foo-16        0              0              +0.00%
Benchmark_LineFilter/simplified_false_(?i)foo-16     1              0              -100.00%

benchmark                                            old bytes     new bytes     delta
Benchmark_LineFilter/default_true_(?i)foo-16         0             0             +0.00%
Benchmark_LineFilter/simplified_true_(?i)foo-16      128           0             -100.00%
Benchmark_LineFilter/default_false_(?i)foo-16        0             0             +0.00%
Benchmark_LineFilter/simplified_false_(?i)foo-16     128           0             -100.00%
```

It's not much but for a billions line it makes a big difference.

Signed-off-by: Cyril Tovena <cyril.tovena@gmail.com>

* typo

Signed-off-by: Cyril Tovena <cyril.tovena@gmail.com>

* Simplified version

Signed-off-by: Cyril Tovena <cyril.tovena@gmail.com>

* Cleanup

Signed-off-by: Cyril Tovena <cyril.tovena@gmail.com>

pkg/logql/log/filter.go

@@ -5,6 +5,8 @@ import (
 	"fmt"
 	"regexp"
 	"regexp/syntax"
+	"unicode"
+	"unicode/utf8"
 
 	"github.com/prometheus/prometheus/pkg/labels"
 )
@@ -242,11 +244,47 @@ type containsFilter struct {
 	caseInsensitive bool
 }
 
-func (l containsFilter) Filter(line []byte) bool {
-	if l.caseInsensitive {
-		line = bytes.ToLower(line)
+func (l *containsFilter) Filter(line []byte) bool {
+	if !l.caseInsensitive {
+		return bytes.Contains(line, l.match)
 	}
-	return bytes.Contains(line, l.match)
+	if len(l.match) == 0 {
+		return true
+	}
+	if len(l.match) > len(line) {
+		return false
+	}
+	j := 0
+	for len(line) > 0 {
+		// ascii fast case
+		if c := line[0]; c < utf8.RuneSelf {
+			if c == l.match[j] || c+'a'-'A' == l.match[j] {
+				j++
+				if j == len(l.match) {
+					return true
+				}
+				line = line[1:]
+				continue
+			}
+			line = line[1:]
+			j = 0
+			continue
+		}
+		// unicode slow case
+		lr, lwid := utf8.DecodeRune(line)
+		mr, mwid := utf8.DecodeRune(l.match[j:])
+		if lr == mr || mr == unicode.To(unicode.LowerCase, lr) {
+			j += mwid
+			if j == len(l.match) {
+				return true
+			}
+			line = line[lwid:]
+			continue
+		}
+		line = line[lwid:]
+		j = 0
+	}
+	return false
 }
 
 func (l containsFilter) ToStage() Stage {
@@ -269,7 +307,7 @@ func newContainsFilter(match []byte, caseInsensitive bool) Filterer {
 	if caseInsensitive {
 		match = bytes.ToLower(match)
 	}
-	return containsFilter{
+	return &containsFilter{
 		match:           match,
 		caseInsensitive: caseInsensitive,
 	}

pkg/logql/log/filter_test.go

@@ -10,6 +10,7 @@ import (
 func Test_SimplifiedRegex(t *testing.T) {
 	fixtures := []string{
 		"foo", "foobar", "bar", "foobuzz", "buzz", "f", "  ", "fba", "foofoofoo", "b", "foob", "bfoo", "FoO",
+		"foo, 世界", allunicode(), "fooÏbar",
 	}
 	for _, test := range []struct {
 		re         string
@@ -53,6 +54,8 @@ func Test_SimplifiedRegex(t *testing.T) {
 		{".*||||", true, TrueFilter, true},
 		{"", true, TrueFilter, true},
 		{"(?i)foo", true, newContainsFilter([]byte("foo"), true), true},
+		{"(?i)界", true, newContainsFilter([]byte("界"), true), true},
+		{"(?i)ïB", true, newContainsFilter([]byte("ïB"), true), true},
 
 		// regex we are not supporting.
 		{"[a-z]+foo", true, nil, false},
@@ -95,6 +98,14 @@ func Test_SimplifiedRegex(t *testing.T) {
 	}
 }
 
+func allunicode() string {
+	var b []byte
+	for i := 0x00; i < 0x10FFFF; i++ {
+		b = append(b, byte(i))
+	}
+	return string(b)
+}
+
 func Test_TrueFilter(t *testing.T) {
 	empty := []byte("")
 	for _, test := range []struct {
@@ -169,13 +180,21 @@ func benchmarkRegex(b *testing.B, re, line string, match bool) {
 	b.ResetTimer()
 	b.Run(fmt.Sprintf("default_%v_%s", match, re), func(b *testing.B) {
 		for i := 0; i < b.N; i++ {
-			m = d.Filter(l)
+			for j := 0; j < 1e6; j++ {
+				m = d.Filter(l)
+			}
 		}
 	})
 	b.Run(fmt.Sprintf("simplified_%v_%s", match, re), func(b *testing.B) {
 		for i := 0; i < b.N; i++ {
-			m = s.Filter(l)
+			for j := 0; j < 1e6; j++ {
+				m = s.Filter(l)
+			}
 		}
 	})
 	res = m
 }
+
+func Test_rune(t *testing.T) {
+	require.True(t, newContainsFilter([]byte("foo"), true).Filter([]byte("foo")))
+}